Progesterone receptor is one of a group of steroid hormone receptors that has been shown to have important regulatory effects in mammary and other target tissues. The qualitative and quantitative determination of progesterone receptor is useful in, for example, the therapeutic management of breast cancer. Currently, available assays for progesterone receptor have drawbacks that make them difficult to use in most clinical laboratories. The major longterm goals of this proposal are the development of a sensitive, reliable enzyme linked immunoassay for progesterone receptor, the production of monoclonal antibodies capable of distinguishing between the various forms of the receptor, and the isolation of purified human progesterone receptor using an immunoaffinity resin prepared by using one of the above monoclonal antibodies.